Where can I find a proper API reference?
Look for class members, (properties, methods et al), in the classes doc page.

bpy.data is  a reference to an instance of the bpy.types.BlendData class. All the members of that class will be found in the documentation page for the  class (or type),   

Most python documentation, I am familiar with, is organized this way, and has often, as is the case here generated from the source.

There may be many references , in this example scrolling down to reference section see that context.blend_data is also  an instance of bpy.types.BlendData.  Note Even though context.blend_data and bpy.data are both of the same type and for this case mostly the same object, but it is not necessarily so. Might start using me = context.blend_data.meshes.new(...) for methods where context is passed.

A type with numerous references is the  blender object. bpy.types.Object for instance the context object of any blender object type. The target of a driver, modifier, constraint on an object. A member of a collection: blend data objects; collection objects or scene objects  etc etc etc etc.

Blender uses the convention: names of properties and methods are in lower case, classes in title case. eg bpy.types.WindowManager is a class (being in bpy.types is a bit of a give away too), bpy.context.window_manager an instance of it.

Turn on developer extras to get an API link by right clicking on a property.